To a narcissist, I would recommend a functional approach, along the following lines:
Know and accept thyself. This is who you are. You have good traits and bad traits and you are a narcissist. These are facts. Narcissism is an adaptive mechanism. It is dysfunctional now, but, once, it saved you from a lot more dysfunction or even non-function. Make a list: what does it mean to be a narcissist in your specific case? What are your typical behaviour patterns? Which types of conduct do you find to be counterproductive, irritating, self-defeating or self-destructive? Which are productive, constructive and should be enhanced despite their pathological origin?
Decide to suppress the first type of behaviours and to promote the second. Construct lists of self-punishments, negative feedback and negative reinforcements. Impose them upon yourself when you have behaved negatively. Make a list of prizes, little indulgences, positive feedbacks and positive reinforcements. Use them to reward yourself when you adopted a behaviour of the second kind.
Keep doing this with the express intent of conditioning yourself. Be objective, predictable and just in the administration of both punishments and awards, positive and negative reinforcements and feedback. Learn to trust your "inner court". Constrain the sadistic, immature and ideal parts of your personality by applying a uniform codex, a set of immutable and invariably applied rules.
Once sufficiently conditioned, monitor yourself incessantly. Narcissism is sneaky and it possesses all your resources because it is you. Your disorder is intelligent because you are. Beware and never lose control. With time this onerous regime will become a second habit and supplant the narcissistic (pathological) superstructure.
You might have noticed that all the above can be amply summed by suggesting to you to become your own parent. This is what parents do and the process is called "education" or "socialisation". Re-parent yourself. Be your own parent. If therapy is helpful or needed, go ahead.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase: http://www.narcissistic-abuse.com/thebook.html)